A few hours before his arrest, King County prosecutors filed second-degree murder charges against Cosgaya-Alvarez, a self-described member of the Sureno Playboys gang. Prosecutor's spokesman Dan Donohoe said the youth was charged as an adult.

The fatal shooting happened about 8:30 p.m. Thursday at Lakota Middle School in Federal Way as boys were leaving a recreation league football practice.

The girlfriend of a man in the Lincoln Navigator with Cosgaya-Alvarez identified her boyfriend, according to court documents. Police arrested him in Kent on Friday night, and that 19-year-old man identified Cosgaya-Alvarez and a third teen who was driving, according to court documents.

Officers found the driver and the Lincoln Navigator at his parents' home late Friday night. The teen told police that Cosgaya-Alvarez bought the gun used in the shooting illegally earlier this month in downtown Kent.

On Saturday, police took the unusual step of releasing the teen's name and photo, saying he was armed and dangerous.

Cosgaya-Alvarez often goes by nicknames "Youngster" or "Junior" and has tattoos on his arms that read "Distrito Federal," according to police.

"The willingness to put young people and their families in jeopardy -- that's what brought us to the conclusion to release the picture," Schrock said. "We were sure that we had the shooter."

The photograph was obtained from another law enforcement agency for whom Cosgaya-Alvarez was a "field contact," Schrock said. Such contacts sometimes know about crimes, and officers routinely talk to them.

"It's amazing how helpful field interviews are in solving serious crimes," she said. "Sometimes officers say, 'Let me see your tattoos.' They're kind of proud to show them to you, which ultimately in some cases works against them in the end."

Court records show that Cosgaya-Alvarez was jailed in September and accused of assaulting a police officer. No charges were filed in the case, and Cosgaya-Alvarez was released. He was arrested on suspicion of illegal gun possession in April. That arrest also failed to result in charges.

The same day that Cosgaya-Alvarez's photograph was circulated in the news media, he left a phone message for a Federal Way police detective saying he wanted to talk about the case and left his dad's cell phone number.

The teen's father told a Federal Way officer on Saturday that his son confessed to being at the scene, but said he was scared after Mendez started the altercation, according to court documents.

The boy told relatives that Mendez fired two shots at him -- a claim police say the evidence doesn't support.

Cosgaya-Alvarez's father didn't tell police if his son had admitted to the killing.

Schrock didn't say how police learned the teenager was in West Seattle, but the Seattle police helped in making the arrest.